As semiconductor devices continue to scale to smaller and smaller dimensions, capacitors used in integrated circuits, such as those used in DRAM storage cells, need higher capacitance/unit area. One proposed way to accomplish this is to switch from the conventional polysilicon-insulator-polysilicon capacitors to metal-insulator-metal (MIM) capacitors, such as the one shown in FIG. 1. In the MIM capacitor 10, metals used as the top 12 and/or bottom 14 electrodes are those with low oxygen affinity such as W, Ni, Co, Ir, Ru, and Pt. The insulator 16 is a metal oxide using metals that have a high oxygen affinity. Exemplary metal oxides include Ta2O5, Nb2O5, Al2O3, Y2O3, HfO2, TiO2, BaO, SrO and the so-called BST (BaxSr1-xTiO3, 0&lt;.times.&lt;1). These metal oxides have dielectric constant values ranging from 10 to 1000 (depending on material, microstructure, crystalline phase, and crystal orientation).
Currently, Ta2O5 and BST are considered the most promising capacitor materials for 256 Mbit and 1 Gbit DRAMs. Ta2O5 and BST can be deposited through reactions of metal-organic precursors and oxygen and annealed in the O2 or UV-O3 ambient. Unfortunately, due to their low oxidation resistance, W, Ni and Co are easily oxidized in the O2 ambient at the high temperatures required. When trying to deposit Ta2O5 on W, the W oxidizes to form WO3-y which can vaporize and result in a nonuniform dielectric. Moreover, Ta2O5-y can be produced instead of Ta2O5. Ta2O5-y is not stociometric and results in reduced capacitance due to leakage. Although Ir, Ru, and Pt have very high oxidation resistance, they are very expensive and difficult to etch.
One prior art method of selective oxidation for poly-metal gate formation was proposed by Kobayashi et al (Proc. of 15.sup.th Conf. Solid State Devices and Material p. 217 (1983)). In this method, a wet hydrogen oxidation procedure was developed to allow the silicon to oxidize while leaving the tungsten unaffected in a post gate-etch oxidation. The method is based on thermodynamic calculations which show that at, for example, 1000.degree. C. and a P(H2O)/P(H2) ratio (partial pressure ratio of H2O and H2) of 1.0e-05, the equilibrium: EQU Si+2 H2O.rarw..fwdarw.SiO2+2H2
prefers the right side of the reaction, i.e., oxidation of Si and EQU W+3H2O.rarw..fwdarw.WO3+3H3
prefers the left side of the reaction, i.e., reduction of WO3 to W. Therefore, under appropriate conditions, it is possible to oxidize silicon again such that the oxidation rate of W will be prevented.
Unfortunately, it is difficult to generate a uniform steam of pure H2O without heavy metal contaminants because it is hard to completely remove heavy metal from the clean and deionized water used in steam generators for semiconductor device manufacturing. It is also dangerous to generate and control a proper H2/H2O gas ratio using a burning process of mixed oxygen and hydrogen under an excessive hydrogen environment.
The above process has been proposed for light thermal oxidation. In the area of non-selective CVD-SiO2, several CO2-H2 gas chemistries have been proposed. One such gas chemistry is SiH4-CO2-H2 and another is SiH2Cl2-CO2-H2. A CO2-H2 gas chemistry has also been used to produce H2O for H2O addition reactions such as 2AlCl3+3H2O.fwdarw.Al2O3+6HCl. In addition, CO gas has been used in metallurgy as a strong reduction reagent of metal oxides for metal production.